1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus such as a video tape recorder (VTR), and more particularly, to a magnetic recording and reproducing apparatus capable of high-speed changeover.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a relevant portion of a magnetic recording and reproducing apparatus as shown by Japanese Laid-open Patent Application No. S63-108502. In this prior art, a magnetic head apparatus 50 used for both recording and reproduction has its one end connected to a power line 51 of a voltage Vcc and has its other end connected to the collector of a recording current supplying transistor 52. The both ends of the magnetic head apparatus 50 are further connected to input terminals of a reproduction input amplifier 53 comprising a differential amplifier. Reference numerals 54 and 55 represent differential transistor.
Reference numerals 56 and 57 represent recording/reproduction changeover switches. During recording, the switch 56 is ON and the switch 57 is OFF. During reproduction, the switch 56 is OFF and the switch 57 is ON. Since the direct current impedance at a coil L of the magnetic head apparatus 50 is low, the voltage at the collector side of the transistor 52, i.e. at a point a is a power source voltage Vcc for a circuit. Since the differential transistor 54 is activated by supplying a bias voltage from another bias source 59 by way of a resistor 60 to its base, the direct current voltages at the points a and b are different. For this reason, the points a and b are cut off from each other by a capacitor 58 with respect to direct currents.
For this reason, the changeover between recording and reproduction takes a long time in this prior art. For example, when a recording current flows into the magnetic head apparatus 50, at the point a, a recording voltage is generated with the direct current voltage Vcc at the point a as the center as shown in FIG. 2. If the recording voltage is cut off in the middle at a point c or d, the voltage varies until it returns from the voltage at the point to the voltage Vcc. At this time, since the capacitor 58 is present, the activation of the reproduction input amplifier 53 is delayed according to the time constant for the charging and discharging of the capacitor 58.
Since it is considered that the prior art of FIG. 1 is originally intended for a changeover on the order of approximately 60 .mu.s, the speed of the changeover can be comparatively low. However, this prior art cannot be used when a high-speed changeover is required. For example, there is a format as shown in FIG. 3 in which different pieces of information A, B and C are recorded on each block area of each recording track 90 of a magnetic tape 91 and blank spaces G1 and G2 are provided therebetween. In this format, assuming that B is audio information, that C is video information and that an after recording of the video is performed, recording must be performed during the period of the information B and reproduction must be performed during the period of the information C.
At this time, the changeover from recording to reproduction must be made during the period of G2. Therefore, the time which can be used for the changeover is very short: at most 40 .mu.s. Since the periods of G1 and G2 must be reduced as much as possible to increase the quantity of the information A, B and C, the changeover time is desirably on the order of approximately 10 .mu.m. The above-described prior art can hardly be used in such a case.